Generally, semiconductor device manufacturers fabricate many semiconductor devices within a single semiconductor substrate. Thus, a variety of techniques have been developed to electrically isolate one semiconductor device from another semiconductor device in the semiconductor substrate. For example, semiconductor devices manufactured in compound semiconductor substrates have used oxygen ion (O.sup.+) implants to make regions between active device regions electrically nonconductive. An important aspect in the isolation of compound semiconductor devices is the sequence of the semiconductor device fabrication steps. More particularly, fabrication steps using a self-aligned source/drain implant include forming a gate electrode in an active device region wherein the gate electrode extends over a field oxide to form a gate pad. Subsequently, a dopant is implanted into the active region to form the source/drain regions, followed by an anneal at a temperature of approximately 850.degree. C. Then, isolation regions are formed in the compound semiconductor substrate by implanting oxygen ions (O.sup.+) through the portions of the field oxide not covered by the gate electrode material and annealing the semiconductor substrate at a temperature of approximately 550.degree. C. Thus, conductive regions are implanted with oxygen ions and become non-conductive, thereby forming isolation regions.
A drawback of this technique is that the oxygen isolation implant must occur after the formation of the gate electrode to preclude degradation of the effectiveness of the isolation implant by the 850.degree. C. source/drain implant anneal. Since the region of the substrate under the gate electrode material in the field oxide region is not implanted with oxygen ions, it remains conductive. Thus, a capacitance is present between the gate pad and the substrate which degrades circuit speed and increases power consumption. In addition, connecting gates of different devices with a gate electrode material that is over the field oxide may result in semiconductor devices becoming shorted via the conductive region of the substrate under the gate electrode material. Another drawback is that separate anneals are required for the source/drain implant and for the isolation implant.
Accordingly, it would be advantageous to have a method of isolating regions in a compound semiconductor device that decreases a gate pad capacitance, reduces leakage currents, and reduces the number of processing steps required for fabricating the compound semiconductor device.